Upward acting or vertical opening sectional doors, as well as single panel doors, are ubiquitous as both residential and commercial garage doors. One continuing and pressing need in the art of such doors has been to reduce the weight of the door while not sacrificing strength and rigidity and to provide a suitable secure closure over the door opening, such as a garage vehicle entry opening. One problem associated with providing lightweight doors constructed of metal sheet, as well as polymer materials has been the provision of suitable brackets for transferring loads from door counterbalance or lifting cables to the door structure itself and to also provide a suitable guide member support structure for the lowermost guide member for guiding the door for movement between open and closed positions. Such brackets should also be arranged in such a way as to minimize inadvertent disconnection of the bracket from the door when the door counterbalance or lifting cables are under high tension, so as to minimize the chance of injury, damage to the door structure and/or damage to the counterbalance mechanism. It is to these ends that the present invention has been developed.